1. Field of the Invention
The present invention relates to a rotating shaft to which a needle is attached that indicates numeric characters of a dial in a meter device of a meter of a vehicle or the like, and more particularly to a needle attaching structure of a rotating shaft in which when the needle is pressed-in to the rotating shaft, a stress is not concentrated to a part, so that the needle can be integrally attached.
2. Description of the Related Art
In such a kind of meter device to be mounted on the vehicle or the like, ordinarily, the dial in which characters, numeric characters and a scale are provided is lighted from a back surface side by an exclusive light source or the needle for indicating the characters or the numeric characters of the dial is lighted (for instance, see patent literature 1).
Specifically, as such a meter device, as shown in FIG. 9, for instance, a meter device is known that includes in a meter box 102 under a dial 101, a light source 103, a needle shaft 104 that guides a light from the light source 103 and having a needle 105 attached integrally to an upper end part, bearings 106 and 107 for supporting the needle shaft 104 so as freely rotate, a gear 108 to which a turning force form a motor not shown in the drawing is transmitted to rotate the needle shaft 104 and the needle 105 and a parabolic shaped reflector 109 provided in a part of a light guide path in which the light from the light source 103 is guided in the needle 105, and the needle 105 is lighted by the emission of the light from the light source 103. In the drawing, reference numeral 110 designates a substrate.
Ordinarily, in order to provide a light guide property in the needle shaft or the needle, the needle shaft or the needle is formed with a suitable resin material and the needle is pressed-in to the needle shaft and integrally attached thereto. Accordingly, when there is a large unevenness in working accuracy between these parts, when the needle is pressed-in to the needle shaft, since the needle is loosely attached to the needle shaft, the needle is easily slipped out from the needle shaft. On the contrary, when the needle is tightly attached to the needle shaft, the needle may be occasionally hardly attached to the needle shaft. Thus, a needle attaching structure is proposed in which parts can be assuredly attached without requiring a highly accurate work (for instance, see patent literature 2).
Namely, as shown in FIGS. 10 and 11, in the needle attaching structure, are provided a shaft part 201 made of a synthetic resin and a needle attaching part 202 formed in an upper end part of the shaft part 201 and a needle not shown in the drawing is attached to the needle attaching part 202. In the needle, a tubular attaching fitting is provided in a bottom surface (a lower surface) side as a base end. An inner part of the attaching fitting forms a cylindrical attaching hole H shown in FIG. 12. In FIGS. 11 and 12, reference numeral 204A designates a corner part of a below-described fitting part 204. 204B designates a side part of the fitting part 204. 205 designates a gear.
The needle attaching part 202 has a tapered part 203 formed in an upper end part side and the straight and substantially regular square pole shaped fitting part 204 formed in a lower part side. The tapered part 203 is formed so as to be smoothly fitted to an attaching hole H of the needle and has a substantially regular square pyramid shape which is gradually thicker toward a lower side. An outer peripheral part 203A in an upper end and each corner part 203B are chamfered. On the other hand, in the fitting part 204, as shown in FIG. 12, a length s between opposed sides is formed to be slightly smaller than the inside diameter r of the attaching hole H of the needle and a length t of each diagonal line is formed so as to be slightly larger than the inside diameter r of the attaching hole H of the needle.    Patent literature 1: German Patent Application Laid-Open No. 19538547    Patent literature 2: JP-UM-A-1-120684
However, in such a needle attaching structure, when a section of the fitting part 204 in the shaft part 201 is a simple polygonal form such as a square form, a space is largely formed between the shaft part and the fitting part. As a result, a large quantity of light leaks from the space, so that a rate of illuminating light guided to reach the needle is greatly reduced to deteriorate a light emitting efficiency. Especially, when a section of the fitting part has a triangular form, the space is so large as not to be neglected, so that a rate of the leakage of the light is increased. Therefore, a needle attaching structure has been desired to be developed that can efficiently transmit the light and realize a light emission and illumination with a good light emitting efficiency.
Further, in such a needle attaching structure, when the fitting part having a section of, for instance, a triangular form is pressed-in to the attaching hole H of the needle, since a difference between levels of press-in loads of contact parts in corner parts respectively is larger than that of non-contact parts of the corner parts which do not respectively come into contact with the attaching hole H, there is a fear that cracks are generated due to a stress concentration in the corner parts respectively. The above-described needle attaching structure has a problem in view of the above-described point.